The contrast method of autofocus is generally used in home camcorders. In the contrast method, higher frequency components of image signals within a certain range (focus area) among the image signals (luminance signals) that have been obtained from an image detecting element are integrated in order to obtain a focus evaluation value. Focus adjustment is then automatically performed by moving an imaging lens by maximizing the focus evaluation value. In this manner, the best focus position (i.e., the in-focus position) of an imaging lens, that is, the position that provides the sharpest (i.e., highest) contrast of an image formed by the imaging lens, can be obtained. However, the contrast method searches for the best focus position while moving focusing lens elements in what is called a hill-climbing mode, which has the disadvantage of providing a slow rate of focusing.
In order to provide faster focusing, autofocus systems and methods wherein the current focus state (front focus, back focus and in-focus) of an imaging lens is detected in order to control the focusing by using multiple image detecting elements with different optical path lengths have been proposed in order to resolve the drawback of the slow rate of focusing in the contrast method. These autofocus systems detect the focus state at the image detecting plane where an image detecting element for image production is located, and they control the focus by positioning a pair of focus state detecting elements equidistant in front of, and in back of, positions that are conjugate to the light receiving surface of the image detecting element for image production. The pair of focus state detecting elements provide quantitative focus evaluation values indicative of the focus state at each of the pair of focus state detecting elements. A comparison of the magnitude of those focus evaluation values provides information about the focus state at the light receiving surface of the image detecting element for image production. However, conventional autofocus systems have the drawback that, if the deviation from the in-focus position is too large, the difference in the focus evaluation values obtained from the pair of focus state detecting elements disappears, resulting in the focus state not being detected. Of course, this prevents fast focusing of the imaging lens to the in-focus position from being achieved.
Recently, as high-definition broadcasts have become generally available, the performance requirements of imaging lenses and cameras that are used for these camera systems have become very high. On the other hand, in the actual use of such imaging lenses and cameras, a cameraman adjusts the focus by relying upon his naked eye with reference to an image in a viewfinder, and the resolution in the viewfinder and the resolution of the image viewed in the viewfinder by the naked eye have limitations. Therefore, it is difficult for a cameraman to determine the in-focus position. Even when the cameraman believes that an in-focus image has been recorded, it is not unusual to discover that the image is not in-focus when the recorded image is played back on a large screen monitor. Therefore, the need for an autofocus system that can accurately and reliably detect the in-focus state, which state cannot accurately and reliably be detected by manual focusing using a viewfinder, keeps increasing.
However, a conventional broadcasting camera does not generally include an internal autofocus system, nor are autofocus systems always preferably used in all broadcasting cameras in all situations. In fact, it is not practical to include an autofocus system in all cameras. Therefore, many cameras now do not include, and many cameras of the future will not include, an autofocus system, even though those cameras are and will be used in many situations where use of an autofocus system would be advantageous.